The invention relates to valves of the type including a spring for normally biasing a metallic ball into sealing engagement with both a hard (metallic) seat and a soft (plastic) seat. More particularly, the invention relates to valves of the type including a passageway adapted for receiving a stinger mechanism to unseat the ball and thereby enable fluid to be injected through or vented from the valve body.
Valves of the present invention may be broadly defined as check valves, i.e., such valves are normally held in the closed position by spring pressure acting on a ball and/or fluid pressure on the ball-side or system pressure-side of the metallic seating surface. A device commonly referred to as a stinger may be utilized to engage the ball and forcefully unseat the ball from the seating surfaces in order to accomplish the desired operation. Such valves have a variety of uses, including the release of fluid (or fluid pressure) from an interconnected assembly, the equalization of fluid pressure at both ends of the valve body, and/or the injection of fluid past the ball and into any number of assemblies to which the valve may be connected. For exemplary purposes, the valve of the present invention may be used to facilitate the installation or removal of a gate valve in a tubing head under pressure. It should be understood, however, that the valve of the present invention may be utilized with numerous devices or assemblies to relieve pressure, equalize pressure, and/or inject fluid into such devices and assemblies.
Biased spherical sealing members have certain advantages over other types of sealing members in valve bodies, such as conical-shaped members or "gate-type" sealing members. For example, ball-type sealing members allow the sealing surface of the member to repeatedly change, thereby increasing the effective life of the sealing member. Also, valves employing such ball-type sealing members enable the seal to be easily unseated by a stinger mechanism to relieve pressure from the valve.
Metallic or "hard" seating surfaces in a valve generally provide a good seal with a metallic ball when the valve is subjected to high fluid pressure. At low fluid pressure, however, the metal-to-metal engagement is frequently not made with sufficient force to inhibit the leakage of fluids, and especially gases, past the seal. Soft or plastic-material seats, on the other hand, generally provide good sealing characteristic with a metallic ball if the valve is only utilized at low pressures, although the fluid in the valve may tend to cause deterioration of the soft seat. When a soft seat valve is subjected to both relatively low and high pressures, however, soft seat material deformation or "flow" under high pressure frequently results in the subsequent loss of a low pressure seal.
In an attempt to obtain the benefits of both a hard and a soft seat, valves have been devised to include both metallic and plastic seats for sealing engagement with a metallic ball. As disclosed, for example, in U.S. Pat. No. 4,347,915, the grease fitting includes a soft seat for sealing under relatively low fluid pressure, and a hard or metal seat for sealing under relatively high fluid pressure. Although the seating technique disclosed in the above-referenced patent has advantages over a valve with either a soft seat or a hard seat alone, it suffers from certain drawbacks which limit its acceptability in the industry, especially when such a seating arrangement is utilized in a valve designed for a stinger operation to unseat the ball from the hard and soft seat.
When a stinger is utilized with the sealing technique as shown in the above-referenced patent, the soft seat may "blow-out" as the stinger unseats the ball from the metal-to-metal seat. The soft seat/ball seal may be maintained although the metal seat/ball seal is broken, resulting in high fluid pressure acting on the soft seat. This force on the soft seat may, especially under elevated temperatures, cause soft seat material deformation to the extent that the soft seat would be ejected from the valve. If blowout occurs, the benefits of a soft seat obviously have been lost, although the operator may not realize that a blowout has occurred until the entire system is reactivated and a low pressure leakage from the valve is observed. Thus, a valve with the dual soft seat and hard seat seal of the U.S. Pat. No. 4,347,915, when utilized with a stinging operation, may experience unacceptable failure in the field. Also, although the combination of a hard seat and a soft seat may reduce soft seat deformation compared to a soft seal alone, concern for soft seat blowout may result in the selection of a soft seat material having a relatively high resiliency. In other words, material selection of the soft seat to minimize the likelihood of a blowout is made at the expense of soft seat elasticity, thereby reducing the effectiveness of the soft seat seal under low pressure.
The disadvantages of the prior art are overcome by the present invention, and improved techniques and apparatus are hereinafter provided for obtaining an improved check valve of the type adapted for unseating a ball with a stinger.